Speaking as one of the people who has designed SSS Arduinos, I don't think that the updates between Diecimila and Duemillanove are either relevant or interesting for a serial version of the arduino (the main difference is the automatic power switching between USB and external power, which is useless in a serial version.) (an ATmega328 will drop into the existing SSS Arduino designs just fine, even my pre-diecimila version.)